Lambert's cosine law relates energy, angle of incident, and surface area. This tells us that when light hits a surface at an angle, less energy falls on the surface per unit area. This is the idea behind the 4 seasons, and explains why we get more or less sunlight during the seasons due to the tilt of the earth.
In optics, Lambert's cosine law says that the radiant intensity or luminous intensity observed from an ideal diffusely reflecting surface or ideal diffuse radiator is directly proportional to the cosine of the angle ? between the direction of the incident light and the surface normal.
A detailed explanation on Lambert's cosine law https://en.wikipedia.org/wiki/Lambert's_cosine_law